1. Field of the Invention
The present invention relates to a flat display device, and in particular, to a liquid crystal display (LCD) device and a method for driving the same with enhanced screen quality and reduced cross-talk effect.
2. Description of the Related Art
In general, an LCD device is manufactured by facing two glass substrates (also called top and bottom plates) and injecting liquid crystal in the space between the two glass substrates. A data line and a gate line are arranged in a matrix form at the bottom plate to define a plurality of pixel regions. A thin film transistor and a pixel electrode are aligned in each pixel region.
Provided at the top plate are a common electrode for applying a common voltage to the liquid crystal, and a color filter layer for expressing colors of red, green and blue (R, G, B).
The following is a detailed description of various components formed on the bottom plate and the top plate.
As shown in FIG. 1, a plurality of gate lines 12 are formed on a transparent substrate 11 made of glass or quartz, and a plurality of data lines 13 are formed in a direction crossing the gate lines 12. A pixel region is defined by each data line 13 and gate line 12. A pixel electrode is aligned in the pixel region, and a thin film transistor is formed at a point where the data line 13 crosses the gate line 12.
A black matrix layer 14 is formed in a net shape on a transparent substrate 11a for shielding penetration of light to various portions of the device except the pixel electrode formed on the bottom plate. A color filter layer 15 is formed between each black matrix to express colors. A common electrode 16 is formed throughout the surface of the transparent substrate 11a including the color filter layer 15 and the black matrix layer 14.
The following is a description of a conventional LCD device made with reference to the accompanying drawings.
FIG. 2 is a diagram illustrating a panel structure of a conventional LCD device, in which a driving circuit section is integrated with a pixel section.
Referring to FIG. 2, the conventional LCD device includes a pixel section 21 having a plurality of gate lines and a plurality of data lines arranged to cross each other and a plurality of pixels having thin film transistors and liquid crystal capacitors formed at each crossing point. A gate driving circuit section 23 applies driving signals to the gate lines in order. A source driving circuit section 25 including a plurality of data line sets applies video signals to each set of data lines. A precharge circuit section 27 precharges the data lines.
The precharge circuit section 27 includes a switching section 27a composed of a precharging voltage terminal Vp and transistors for connecting each data line. The data lines are precharged a predetermined level by means of precharge control signals Cp applied to the transistor gates.
The source driving circuit section 25 includes a plurality of data line sets, each set being composed of n number of data lines. Video signal lines S1, S2, . . . , Sn are connected to each set of data lines so that the video signals are applied to the corresponding set of data lines by means of n number of control signals C1, C2, . . . , Cn.
A driving method of the conventional LCD device constructed as discussed above will now be described with reference to FIG. 3 illustrating a driving waveform.
If a gate driving signal is applied as shown in FIG. 3, the precharge circuit section 27 precharges each data line to a predetermined level with an intermediate voltage between a positive field and a negative field of the video signals.
Thereafter, each set of data lines of the source driving circuit section 25 is activated, thereby applying the video signals to the data lines. Here, the control signals C1, C2, . . . , Cn are activated in order. When the control signals C1 is activated, the other control signals C2, C3, . . . , Cn remain inactive. When C1 becomes inactive, the control signal C2 changes to an active state.
Each set of data lines become active through the above process, and video signals are thus applied to the corresponding data lines.
Assuming that the video signals in a positive field have a voltage range of about 6-10V and the video signals in a negative field have a voltage range of about 1-5V, the data lines are precharged to a voltage level of about 5.5V.
When the control signal C1 changes to an inactive state and the control signal C2 changes to an active state after the corresponding data lines are applied with video signals, a parasitic capacitance is generated because the video signals are applied to the data lines in order.
The parasitic capacitance causes distortion of the video signals applied to the data lines. Therefore, if the control signals Cn become active, the video signals applied to the corresponding data line are significantly distorted.
Such a distortion of the video signals is attributable to the parasitic capacitance generated between the two data lines adjacent to a given liquid crystal capacitor. Thus, it is crucial to reduce the parasitic capacitance. However, reduction of the parasitic capacitance has a limit as it is closely related to an aperture rate. This effect will now be described with reference to FIG. 4.
FIG. 4 is a diagram illustrating the generation of the parasitic capacitance between two adjacent data line and a liquid crystal capacitor between them in the conventional LCD device and the driving method.
Referring to FIG. 4, a capacitance is generated between the data lines adjacent to the liquid crystal capacitor within a pixel. To be specific, if a video signal is applied to an mth data line D_m and then to an m+1th data line D_m+1, a coupling is generated due to a parasitic capacitance Cdpm between the liquid crystal capacitor CLC and the mth data line. The coupling is also generated due to a parasitic capacitance Cdpm+1 between the liquid crystal capacitor CLC and the m+1th data line.
Thus, the conventional LCD device and its associated driving method described above pose a problem by generating a coupling, which is attributable to a parasitic capacitance between a liquid crystal capacitor and adjacent data lines due to the video signals applied to the data lines consecutively.
The coupling is shown in the form of a vertical cross talk. In other words, when there is a difference in a screen pattern, a value of the liquid crystal capacitor is changed and the coupling voltage is changed due to the parasitic capacitance, thereby reducing the screen display quality.